Phase shifters are known for adjusting the phase of electrical signals in various kinds of products and systems. Phase shifters are especially useful in navigation, tracking and communication equipment to control characteristics of the associated electrical signals. Various types of phase shifters have been designed for particular uses, but while useful in particular environments, the disadvantages of many phase shifter designs have limited their use in the field of multi-carrier, high power antennas, such as base station antennas as used in the mobile communications industry.
One conventional technique is the line-stretcher phase shifter which uses a coaxial transmission line that is extendable in a telescope-type fashion. This technique usually requires rather complex sliding-contacts and can be very sensitive to corrosion. Another conventional technique is a phase shifter that is adjusted mechanically by sliding an external sleeve along the body of the phase shifter so to alter the relative phase of the signals at the phase shifter's outputs. A drawback of this type phase shifter that employs moveable or sliding contacts is that it is susceptible to generating adverse Passive Intermodulation (PIM) that occurs especially when high power and multi-carrier electromagnetic energy is directed over metal contacts.
Solid state electronics, such as varactor diodes, have been used to achieve phase shifting without the problems associated with mechanical shifters. However, these solid state electronic phase shifting methods are usually not compatible with high power levels due to their inherent nonlinearities, and active solid state solutions require power amplifiers which can be very large and expensive.
Phase shifters employing ferro-magnetic materials (“ferrites”) change the phase of a signal in a feed line by applying a direct current magnetic field to the feed line. However, ferrite phase shifters can be very large, heavy, and expensive. While recently developed thin-film techniques have reduced their size to some extent, such ferrite phase shifters are usually nonlinear at high power levels making them inappropriate for multi-carrier communications operating at high power levels.
Other conventional phase shifting techniques use a mechanical movement of a dielectric material into electrical field lines, but the effective relative phase shift generated can be small for materials with low dielectric constants and hence require large-sized phase shifters for practical applications. For high-dielectric constant materials, a significant impedance mismatch can occur at the interface to the dielectric loaded region, which causes an undesirable return loss. Further, solutions with high dielectric materials are further prone to power loss into dielectric resonant modes. The competing mechanical and electrical demands for phase shifters, especially in constrained environments of many communications systems, makes most of these conventional designs inappropriate to meet the cost, size and performance requirements of certain systems, especially communication system antennas characterized by high power and multi-carrier use.
Consequently, there is a need in the art for a radio frequency (RF) phase shifter and method that is compact, low cost, durable and reliable in repeating phase shifting operation on RF signals, and that can support high power and multi-carrier RF applications. There is a further need for a method and system for producing linear phase shifts in RF Feed Lines that provide for a relatively low return loss, low power loss, while supporting large RF bandwidths and for an apparatus and method of phase shifting that produces little or no adverse PIM signals. A further need exists for a phase shifter and method that are highly reliable and consistent over numerous cycles and where the system can be manufactured with minimal re-tooling in production plants and at a reduced cost.